This invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a process for forming a protective coating on a thermal barrier coating on a gas turbine engine component, in which the protective coating is resistant to infiltration by contaminants present in the operating environment of a gas turbine engine.
Hot section components of gas turbine engines are often protected by a thermal barrier coating (TBC), which reduces the temperature of the underlying component substrate and thereby prolongs the service life of the component. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. Plasma spraying processes such as air plasma spraying (APS) yield noncolumnar coatings characterized by a degree of inhomogeneity and porosity, and have the advantages of relatively low equipment costs and ease of application. TBC's employed in the highest temperature regions of gas turbine engines are often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure. Similar columnar microstructures with a degree of porosity can be produced using other atomic and molecular vapor processes.
To be effective, a TBC must strongly adhere to the component and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion (CTE) between ceramic materials and the substrates they protect, which are typically superalloys, though ceramic matrix composite (CMC) materials are also used. An oxidation-resistant bond coat is often employed to promote adhesion and extend the service life of a TBC, as well as protect the underlying substrate from damage by oxidation and hot corrosion attack. Bond coats used on superalloy substrates are typically in the form of an overlay coating such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), or a diffusion aluminide coating. During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during engine operation, these bond coats form a tightly adherent alumina (Al2O3) layer or scale that adheres the TBC to the bond coat.
The service life of a TBC system is typically limited by a spallation event driven by bond coat oxidation, increased interfacial stresses, and the resulting thermal fatigue. In addition to the CTE mismatch between a ceramic TBC and a metallic substrate, spallation can be promoted as a result of the TBC being contaminated with compounds found within a gas turbine engine during its operation. Notable contaminants include such oxides as calcia, magnesia, alumina and silica, which when present together at elevated temperatures form a compound referred to herein as CMAS. CMAS has a relatively low melting temperature of about 1225° C. (and possibly lower, depending on its exact composition), such that during engine operation the CMAS can melt and infiltrate the porosity within cooler subsurface regions of the TBC, where it resolidifies. As a result, during thermal cycling TBC spallation is likely to occur from the infiltrated solid CMAS interfering with the strain-tolerant nature of columnar TBC and the CTE mismatch between CMAS and the TBC material, particularly TBC deposited by PVD and APS due to the ability of the molten CMAS to penetrate their columnar and porous grain structures, respectively. Another detriment of CMAS is that the bond coat and substrate underlying the TBC are susceptible to corrosion attack by alkali deposits associated with the infiltration of CMAS.
Various studies have been performed to find coating materials that are resistant to infiltration by CMAS. Notable examples are commonly-assigned U.S. Pat. Nos. 5,660,885, 5,773,141, 5,871,820 and 5,914,189 to Hasz et al., which disclose three types of coatings to protect a TBC from CMAS-related damage. These protective coatings are generally described as being impermeable, sacrificial, or non-wetting to CMAS. Impermeable coatings are defined as inhibiting infiltration of molten CMAS, and include silica, tantala, scandia, alumina, hafnia, zirconia, calcium zirconate, spinels, carbides, nitrides, silicides, and noble metals such as platinum. Sacrificial coatings are said to react with CMAS to increase the melting temperature or the viscosity of CMAS, thereby inhibiting infiltration. Suitable sacrificial coating materials include silica, scandia, alumina, calcium zirconate, spinels, magnesia, calcia, and chromia. As its name implies, a non-wetting coating reduces the attraction between the solid TBC and the liquid (e.g., molten CMAS) in contact with it. Suitable non-wetting materials include silica, hafnia, zirconia, beryllium oxide, lanthana, carbides, nitrides, silicides, and noble metals such as platinum. According to the Hasz et al. patents, an impermeable coating or a sacrificial coating can be deposited directly on the TBC, and may be followed by a layer of an impermeable coating (if a sacrificial coating was deposited first), a sacrificial coating (if the impermeable coating was deposited first), or a non-wetting coating. If used, the non-wetting coating is the outermost coating of the protective coating system.
Other coating systems resistant to CMAS have been proposed, including those disclosed in commonly-assigned U.S. Pat. Nos. 6,465,090, 6,627,323, and 6,720,038. With each of these, alumina is a noted candidate as being an effective sacrificial additive or coating, in other words, reducing the impact of CMAS infiltration by reacting with CMAS (being sacrificially consumed) to raise the melting point and viscosity of CMAS. A number of approaches have been considered for applying alumina and other materials capable of inhibiting CMAS infiltration (hereinafter, CMAS inhibitors), including those disclosed by the above-identified commonly-assigned patents. Certain approaches are more effective at placing a CMAS inhibitor into the open porosity within the TBC, while others such as EB-PVD deposition, slurry top coats, and laser glazing tend to be more effective at depositing the CMAS inhibitor as a discrete outer layer on the TBC. In the case of alumina, the approach has generally been to provide alumina in the form of an additive layer overlying the TBC, rather than as a co-deposited additive within the TBC, since solid alumina and zirconia are essentially immiscible and the mechanism by which alumina provides CMAS protection is through sacrificial consumption. Nonetheless, it is desirable to have at least some alumina deposited in the open porosity of a TBC to maintain a level of CMAS protection in the event the alumina layer is breached or lost through spallation, erosion, and/or consumption.
Chemical vapor deposition (CVD) processes have been shown to be capable of being optimized for either higher deposition rates that primarily deposit alumina as a discrete additive layer on the outer TBC surface, or lower deposition rates that promote infiltration of a relatively small amount of alumina into the open porosity of a TBC. Spallation tests with CMAS contamination have indicated that TBC's protected with either approach exhibit similar CMAS resistance, even though those primarily infiltrated with alumina have much lower alumina contents. However, the CVD deposition of alumina with good penetration into the porosity of a TBC generally requires expensive specialized equipment and is typically limited to very low deposition rates.
Another approach capable of infiltrating a TBC with a CMAS inhibitor is liquid infiltration with a precursor of the inhibitor. To be successful, the precursor and any solvents, carriers, etc., used therewith must not damage the TBC, other layers of the TBC system, or the substrate protected by the TBC system. Other key requirements for a successful liquid infiltration approach include achieving an adequate degree of infiltration and depositing an effective quantity of alumina. To promote the latter, the precursor should contain a relatively high level of aluminum that can be converted to yield a known or predictable amount of alumina. For those precursors requiring a solvent or carrier, another important consideration is the solubility of the precursor in its carrier since a precursor with a high conversion efficiency will not be effective if only a small loading of the precursor can be placed into solution.
In view of the above, while various approaches are known for depositing alumina and other CMAS inhibitors, there is an ongoing need for deposition techniques capable of depositing an effective amount of a CMAS inhibitor on and/or within a TBC that will optimize the ability of the inhibitor to prevent damage from CMAS infiltration.